The Data Cube: a 3D Simulation of the Epoch of Reionisation

The Epoch of Reionisation

The Big Bang saw the creation and ionisation of the universe. The ionisation of a particle occurs when the particle becomes charged, essentially meaning it is ‘ready’ to react with other particles. The initial ionisation of the universe occurred when neutral hydrogen gas (a proton and an electron) became ionised (the electrons are stripped, leaving a positively charged proton). This ionised gas went from smoothly distributed, to slowly collapsing in different areas to become the highly structured galaxies and clusters of galaxies we have today.

Astrophysicists classify the initial blanket of gas as a ‘smooth’ universe, compared to what we now live in, which is the ‘clumpy’ universe. Imagine that that layer of gas is a pillow. When you add marbles (stars) into it, it becomes clumpy. The more you add, the more rigid it becomes. This is similar to what happened in our universe, but the fabric of the pillow became the marbles themselves.

In the beginning, only a few stars began to form at a very slow rate, but there was a tipping point – where these stars gave way to a rapid spur of supernova all over the universe, which is what we call the Epoch of Reionisation.

So how did astrophysicists discover this? Well, as the universe expands, the stars move away from us. The light they emit appears to ‘shift’ to the red end of the electromagnetic spectrum. Red Shift acts similarly to the sound of an ambulance that changes in frequency as it speeds past you. But in this case, it’s the light from the star. So by probing (measuring) the distance between two points over time, scientists can understand almost everything about the stars; including their origin.

But it’s not just stars that can be measured by this technique. It’s also pulsars (magnetised neutron stars) and even entire galaxies!

Paul Geil, the animator/developer of the 3D data cube, further explains the concept of Red Shift: “We can look at an object’s spectrum and work out how much that light has elongated due to the expansion of the universe.”

What am I looking at?

The easiest way to understand what this cube demonstrates is to separate it out by colour components. All of the black space within the cube is the ionised hydrogen and the red sponge-like material is the neutral hydrogen (that is yet to be ionised). So throughout the simulation, you can see the red blotched sections contracting into the dark space, leaving white spheres behind. The simulation shows the neutral hydrogen becoming ionised – or turning to black. Towards the end of the simulation you see pale blue spheres representing the galaxies formed at the end of the Epoch of Reionisation.

The Data Cube was developed by a group of researchers at the University of Melbourne. It’s called the Dark-ages Reionisation And Galaxy formation Observables from Numerical Simulations team – or DRAGONS for short. Using a giant data set and complicated programs run by supercomputers, the DRAGONS team was able to develop the simulation, above.

So how big is this cube? This simulation represents 100 megaparsecs of 3D-space; half a billion years in the making, which occurred roughly half a billion years after the Big Bang. Let’s break that down: 100 megaparsecs of 3D-space is equivalent to 330 million light years3 . In other words, 3,000 billion billion km3 of space, over half a billion years!

This data simulation is a huge step in our understanding of the way our universe was formed and continues to expand. It allows scientists and curious members of the public to visually see the formation of first stars and galaxies. But more significantly, it allows us to experience the history of our universe in a 3D, lifelike simulation.